This invention relates to adhesive compositions and adhesives containing thioether groups and bonded substrates using such adhesives.
Epoxy structural adhesives, although widely used for a variety of applications, do have some deficiencies. One limitation of epoxy structural adhesives is their tendency to swell and in some cases degrade when exposed to water and/or solvents. For example, some of the known structural adhesives currently used in ink-jet cartridges absorb up to 10 percent by weight of the liquid ink components. This swelling will result in a drastic reduction in the glass transition temperature of the adhesive, converting the adhesive from a strong, structural adhesive into a weak elastomer that is unable to maintain bond strength. The water and/or solvent penetration into adhesives can also lead to chemical degradation because of reactions with water and/or solvent. The swelling and degradation of the structural adhesive eventually leads to interfacial failure of the adhesive-substrate bond. A highly swollen or degraded adhesive network provides a pathway for other components to migrate to and degrade an adhesive substrate interfacial bond. This mechanism can lead to corrosion of the substrate if corrosive ions migrate through the swollen adhesive.
A conventional method of reducing the sensitivity of an adhesive to swelling by water or solvents is to drive the network forming reaction to very high levels of crosslink density. High crosslink density generally leads to high glass transition temperatures and requires high cure temperatures. These factors can lead to high levels of interfacial stress between the adhesive and substrate if the substrate is a metal, glass or ceramic because the thermal expansion of amorphous organic resins such as epoxy resins is much higher than most metals, inorganic glasses or ceramics. For example, the coefficient of thermal expansion (CTE) for an epoxy resin below its glass transition is generally about 60 ppm/xc2x0 C. The CTE for silicon is 2.6-2.8 ppm/xc2x0 C. If a high modulus epoxy resin is bonded to a silicon die at a high temperature, a significant amount of interfacial stress is built up between the adhesive and silicon die. If the silicon die is large and thin, the result can be undesirable deformation or bowing of the die. If the die is rigid enough not to deform, the residual stress from the CTE mismatch is stored at the interface, weakening the bond.
Two factors that affect the interfacial stress in an adhesive-substrate bond are the CTE mismatch and the modulus of the adhesive. The problems associated with CTE mismatch can be alleviated through addition of a xe2x80x9cflexibilizerxe2x80x9d which lowers the modulus of the epoxy structural adhesive. The flexibilizer reacts, at least to some extent, with the epoxy resin during cure to provide flexibility in the polymer backbone. Typically, such flexibilizers are used to reduce the glass transition temperature, reduce the viscosity, and improve the ductility of the epoxy adhesive. The addition of a flexibilizer reduces the interfacial stress by increasing the rubbery or low modulus range of the adhesive and reducing the glassy or high modulus range of the adhesive. During much of the cooling cycle after the cure, the flexible adhesive will be in the rubbery/low modulus state, where interfacial stress is minimized. However, conventional flexibilizers such as polyalkylene oxide amines and epoxy terminated ethers render such adhesives particularly sensitive to water and/or solvent swelling and degradation.
Epoxy terminated polysulfides are another type of epoxy flexibilizers. While polysulfide polymers are known to have good fuel and moisture resistance, they typically contain chemically and thermally unstable disulfide and formal linkages, which make them unsuitable for many applications.
Another deficiency of epoxy resins is their limited adhesion to certain substrates. Epoxy resins are known to have excellent adhesion to some metals and good adhesion to some plastics. In general, epoxy-based adhesives do not provide good adhesion to non-polar plastics such as acrylonitrile-butadiene-styrene (ABS) polymers. The use of epoxy-terminated liquid polysulfide polymers in epoxy-based adhesives is known to improve adhesion to some types of substrates such as steel. However, epoxy-terminated liquid polysulfide polymers have not been demonstrated to improve adhesion to plastics such as ABS polymers, poly(methyl methacrylate) (PMMA) polycarbonate, polyimide, or silicon.
Thus, there remains a need for epoxy-based adhesives having improved water and solvent resistance and/or improved adhesion to difficult to bond substrates such as those made from ABS, poly(methyl methacrylate) (PMMA), polycarbonate, polyimide, silicon dioxide, and silicon die.
The invention provides epoxy resin adhesives that contain thioether segments incorporated into the crosslinked network. A xe2x80x9cthioether segmentxe2x80x9d is defined as a divalent sulfur atom bonded to two carbon atoms. A segment may contain a sequence of two or more thioether groups bonded together. The thioether segments are incorporated into an epoxy resin adhesive by adding a thioether-containing flexibilizer to the adhesive composition. Generally, the more thioether segments incorporated into the flexibilizer and the fewer oxygen ether groups, the more the adhesives are resistant to swelling or absorption due to water and/or solvent exposure.
In one aspect, the invention provides a curable composition for making an adhesive containing thioether segments comprising a mixture of epoxy resin, catalyst and/or curative, and epoxy reactive thioether-containing compound. Preferred epoxy reactive thioether-containing compounds are the thioether di-epoxides. Preferred thioether di-epoxides include 2-{[3-({2-[(2-{[3-(2-oxiranylmethoxy)propyl]sulfanyl }ethyl)sulfanyl]ethyl}sulfanyl)propoxy]methyl }oxirane; 2({3-[(6-{[3-(2-oxiranylmethoxy)propyl]sulfanyl }hexyl)sulfanyl]propoxy}methyl)oxirane; and 2-({3-[(2-{[3-(2-oxiranylmethoxy)propyl]sulfanyl}ethoxyethoxyethyl)sulfanyl]propoxy}methyl)oxirane.
The epoxy reactive thioether-containing compounds described herein can be used to increase flexibility and reduce interfacial stress of the resulting adhesive but have a minimal negative impact on the water and chemical resistance of the adhesive.
Another aspect of the invention is a method of bonding a substrate comprising the steps of contacting the substrate with an adhesive composition comprising epoxy resin, epoxy reactive thioether-containing compound, and catalyst and/or curative, and curing the adhesive composition. The adhesive compositions of the invention provide adhesives having improved adhesion to substrates including those made from ABS polymers, polyimide, polycarbonate, poly(methylmethacrylate), silicon dioxide, and silicon die.
Another aspect of the invention provides an adhesive comprising the reaction product of epoxy resin, catalyst and/or curative, and epoxy reactive thioether-containing compound. The adhesives of the invention are cured adhesive compositions of the invention.
Other aspects of the invention are articles comprising adhesives comprising epoxy resin, epoxy reactive thioether-containing compound, and a curative bonded to substrates comprising silicon, plastic, metal, or combinations thereof.
Adhesion levels of the adhesives of the invention to plastics such as ABS and polycarbonate are in excess of those seen from epoxy-based adhesives that do not contain sulfur atoms or that contain epoxy-terminated polysulfide polymers. In addition, formulations containing epoxy reactive thioether-containing compounds have improved adhesion to polyimide and silicon dioxide passivated silicon wafer die than adhesives that contain polyalkylene oxide amines such as bis-3-aminopropylpolytetramethyleneoxide. Adhesives of the invention may be low stress and are water and solvent resistant.
The incorporation of thioether segments into structural adhesive networks provides adhesives that are resistant to swelling and degradation by water and other solvents. These adhesives retain the desirable processing characteristics of epoxy adhesives but have significantly improved resistance to swelling and attack by solvents and water. The thioether segment containing adhesives of the invention also demonstrate very good adhesion to a variety of substrates that would otherwise be difficult to bond with a conventional epoxy resin-based adhesive.
Epoxy adhesives are known to adhere to a variety of polar substrates such as glass, ceramics, and metals very well, but do not adhere very well to most plastics and noble metals. The ability to widen the number of substrates that epoxy adhesives adhere to would widen the applications for these materials. The ability of such adhesives to adhere to substrates used in the microelectronics industry, such as silicon dioxide, silicon wafer dies, polyimide film, and noble metals such as gold, and to plastics such as ABS and polycarbonate, makes them very useful.
The addition of thioether segments into adhesive compositions provide a method of flexibilizing the adhesive without having a significant adverse affect on the resistance to swell and attack by water and/or organic solvents or corrosive liquids such as inks. The ability to flexibilize an adhesive is important in developing low stress adhesives. To achieve resistance to swelling by water and/or solvent containing liquids, high Tg-high crosslink density structural adhesive are generally used. However, such adhesives may be brittle and lack desirable adhesive characteristics. The high glass transition temperature/high modulus-adhesive may impose excessive thermal stresses on delicate electronic substrates. A high modulus over the entire range of a thermal cycle such as cooling from a high temperature cure can have the undesirable effect of causing the substrate to deform or bow. The ability to reduce the modulus or flexibilize the adhesive via the addition of flexible segment overcomes the lack of ductility and minimizes interfacial stress. Thioether flexibilizers are able to impart ductility and reduce interfacial stress without significantly increasing the susceptibility to swelling by water and other solvents or corrosive liquids.
The adhesive compositions of the invention contain at least one epoxy-reactive epoxy reactive thioether-containing compound. xe2x80x9cEpoxy reactivexe2x80x9d means that the thioether segments react with the epoxide via an addition reaction or copolymerization reaction and is incorporated into the cured epoxy network. Useful thioether containing compounds which exhibit improved adhesion to plastics and water and/or solvent resistance generally have a molecular weight in the range of from about 320 to about 650. Presently preferred epoxy reactive thioether-containing compounds for use as a flexibilizer in an epoxy resin are thioether di-epoxides. Thioether di-epoxides, for example, are effective flexibilizers that can be used to modify epoxy adhesives but will render them less susceptible to degradation by organic solvents and water than conventional flexible epoxies. Preferred thioether di-epoxides include 2-{[3-({2-[(2-{[3-(2-oxiranylmethoxy)propyl]sulfanyl}ethyl)sulfanyl]ethyl}sulfanyl)propoxy]methyl}oxirane; 2({3-[(6-{[3-(2-oxiranylmethoxy)propyl]sulfanyl}hexyl)sulfanyl]propoxy}methyl)oxirane; and 2-({3-[(2-{[3-(2-oxiranylmethoxy)propyl]sulfanyl}ethoxyethoxyethyl)sulfanyl]propoxy }methyl)oxirane, and combinations thereof. The epoxy reactive thioether-containing compound is present in the adhesive compositions of the invention at levels of from about 10 to about 80, preferably 10 to 55, more preferably 10 to 40 parts by weight.
Useful epoxy resins have the ability to cure with a variety of curatives and catalysts and process conditions to form hard very strong structural adhesives. Useful epoxy resins include those made from bisphenol, novolak, cresol novolak compounds, and other polyfunctional phenolic glycidyl ether epoxy resins. Presently preferred epoxy resins are diglycidyl ethers of bisphenol A, bisphenol F, bisphenol AF, bisphenol S, and combinations thereof. Presently preferred commercially available diglycidyl ethers of bisphenol A include TACTIX 123, DER 332 and 331, from The Dow Chemical Company, Midland Mich; and EPON 828 and RSL 1462, from Shell Chemical Company, Houston, Tex. Epoxy resins are present in the adhesive compositions of the invention in a range of about 20 to 80, preferably 40 to 80, more preferably 50 to 70 parts by weight.
A variety of curatives and catalysts are suitable to cure adhesive compositions containing the thioether precursor resins. As used herein, a xe2x80x9ccurativexe2x80x9d is an epoxy reactive multifunctional material that copolymerizes with the epoxy resins via an addition polymerization and becomes covalently incorporated into cured resin composition and a xe2x80x9ccatalystxe2x80x9d is a component that causes the homopolymerization of the epoxy resin or accelerates the reaction of the epoxy resin with curatives. Epoxy reactive resins such as polyamines may act as both a curative and a catalyst. Frequently used epoxy curatives include multifunctional amines and hydrazides, polyfunctional phenolic curatives, multifunctional carboxycyclic acids, multifunctional mercaptans, and anhydrides. Anionic epoxy resin curatives, such as amines (for example, poly(oxyhydrocarbolene)diamines described in U.S. Pat. No. 4,521,490), are a common class of curatives used in these adhesive compositions. A presently preferred curative for two-part epoxy adhesive formulations is 4,7,10-trioxa-1,13-tridecanediamine. The curative can be present in an amount from 0.75 to 1.6 equivalents of xe2x80x94NH per epoxy equivalent and preferably in essentially stoichiometric amounts. At room temperature, curing takes place in about 6 hours to 7 days or longer.
Thioether-containing adhesive compositions cannot be cationically cured at room temperature, but can be catalytically cured with a variety of other catalysts. Catalytically cured epoxies are sometimes preferred to provide one-part epoxy resins. Various tertiary amines and transition metal complexes can be used as catalysts for the adhesive compositions. Imidazoles such as 2-methyl imidazole, imidazole, or blocked imidazoles are a preferred class of catalysts in these compositions. A presently preferred catalytically cured epoxy formulation contains a catalytic amount of 2-ethyl-4-methylimidazole in conjunction with xe2x80x9cDEH 85xe2x80x9d, a phenolic epoxy curative resin available from The Dow Chemical Company. Catalysts may be present in the adhesive compositions of the invention in an amount of from about 0.1 to about 6 percent by weight.
Various other materials can be added to the composition, as is customary with formulating epoxy compositions, to alter or even improve the characteristics of the uncured or cured adhesive. Such materials include solvents, viscosity modifiers, filler, coupling agents, pigments, dyes, fibers, glass or plastic microbeads or bubbles, plasticizers, and flame retardants, such as antimony trioxide, extenders, toughening agents such as rubber toughening agents, conductive particles, for example, thermally and/or electrically conductive, microwave susceptors, antioxidants, UV stabilizers, and the like. Depending on the desired function of the additive, from traces to 100 percent or more by weight of the additive based on the weight of epoxide group-containing compound in the composition may be used. Preferred coupling agents are epoxy-reactive coupling agents such as glycidoxypropyltrimethoxysilane, aminopropyltrimethoxysilane, and mercaptopropyltrimethoxysilane. A preferred filler is spherical silica. Fumed silicas are generally preferred class of rheology control agents.
Generally, the adhesive compositions of the invention are made by first mixing epoxy resin with the epoxy reactive thioether-containing compound, combining a catalyst or catalyst mixture into the resin, adding any optional additives, and then heating the mixture at a suitable temperature for a suitable amount of time to cure the resin. The temperature and time profile for curing any particular adhesive composition is dependent upon the epoxy resin and catalyst or curative used. The techniques used to determine the appropriate temperature/time profile for curing an adhesive composition is well within the knowledge of those skilled in the epoxy adhesives art.
The adhesive compositions and the adhesives of the present invention can be used for bonding application that require a high degree of water and solvent resistance and/or bonding difficult substrates such as ABS polymers, polyimide, PMMA polymers, polycarbonates, or silicon die. The adhesive compositions and the adhesives of the invention can also be used to bond noble metals such as gold, platinum, palladium, silver, iridium, and combinations thereof. An adhesive composition of the invention is simply applied to substrate or substrates to be bonded, the substrates are joined, and the adhesive composition is thermally cured or crosslinked. For example, an adhesive composition of the invention is applied to a print head or an ink-jet cartridge, the print head and ink-jet cartridge are joined, and the adhesive composition is cured.